A ‘dark’ free-floating black hole may have been discovered in Milky Way

A stellar ghost!


By using a technique called gravitational microlensing, a team of astronomers from the University of California, Berkeley, has discovered a ‘dark’ free-floating black hole in the Milky Way galaxy. This was the first time they found a free-floating black hole by observing the brightening of a more distant star as its light was distorted by the object’s intense gravitational field.

Whether a black hole or a neutron star, the object is the first dark stellar remnant; hence the team dubbed this object a stellar ghost. It has a mass between 1.6 and 4.4 times that of the sun.

According to scientists, the leftover remnant of a dead star must be heavier than 2.2 solar masses to collapse into a black hole. Considering the fact and the object’s mass, scientists caution that the stellar ghost object could be a neutron star instead of a black hole.

Jessica Lu, a UC Berkeley associate professor of astronomy, said, “This is the first free-floating black hole or neutron star discovered with gravitational microlensing. With microlensing, we can probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can’t be seen any other way.”

competing study from Baltimore’s Space Telescope Science Institute (STScI) investigated the same microlensing event. It argued that the compact object’s mass is closer to 7.1 solar masses, indicating that it is unmistakably a black hole.

Hubble Space Telescope image of a distant star
Hubble Space Telescope image of a distant star that was brightened and distorted by an invisible but very compact and heavy object between it and Earth. The compact object — estimated by UC Berkeley astronomers to be between 1.6 and 4.4 times the mass of our sun — could be a free-floating black hole, one of perhaps 200 million in the Milky Way galaxy. (Image courtesy of STScI/NASA/ESA)

Both studies use the same data: photometric measurements of the distant star’s brightening as its light was distorted or “lensed” by the super-compact object and astrometric measures of the shifting of the distant star’s location in the sky as a result of the gravitational distortion by the lensing object.

The photometric data were gathered from the Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA). On the other hand, the astrometric data was obtained from NASA’s Hubble Space Telescope. STScI manages the science program for the telescope and conducts its science operations.

This illustration shows how the gravity of a black hole
This illustration shows how the gravity of a black hole warps spacetime and bends the light of a distant star so that its position is shifted as seen from Earth. This deflection, captured by the Hubble Space Telescope (left foreground), and the associated brightening of the star allow astronomers to discover these otherwise invisible, free-floating objects and calculate their mass and velocity. (Illustration credit: NASA, ESA, STScI, Joseph Olmsted)

Because both microlensing surveys caught the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, for short. The team from UC Berkeley suggests that the object lies between 2,280 and 6,260 light-years (700-1920 parsecs) away, in the direction of the center of the Milky Way Galaxy and near the large bulge that surrounds the galaxy’s central massive black hole. In contrast, the STScI group estimated that it lies about 5,153 light-years (1,580 parsecs) away.

Using additional data from Hubble, scientists found that the change in position of the star as a result of the gravitational field of the lens is still observable ten years after the event.

Analysis of the new data confirmed that OB110462 was likely a black hole or neutron star. The different conclusions are because of different measurements of the relative motions of the foreground and background objects.

The astrometric analysis also differs between the two teams. The UC Berkeley-led team argues that it is not yet possible to distinguish whether the object is a black hole or a neutron star. Still, they hope to resolve the discrepancy with more Hubble data and improved analysis in the future.

 star-filled sky in this Hubble Space Telescope
The star-filled sky in this Hubble Space Telescope photo is located in the direction of the galactic center. While monitoring the sky in search of stars that dramatically brighten, the telescope discovered one (center) that brightened and dimmed between 2011 and 2017. This may have been caused by a foreground black hole drifting in front of the star, gravitationally brightening it and distorting its position relative to background stars. (Image credit: NASA, ESA, Kailash Sahu(STScI), with image processing by STScI’s Joseph DePasquale)

Lu said, “As much as we would like to say it is definitively a black hole, we must report all allowed solutions. This includes both lower-mass black holes and possibly even a neutron star.”

Lam said“If you can’t believe the light curve, the brightness, that says something important. If you don’t believe the position versus time, that tells you something important. So, if one of them is wrong, we must understand why. Or the other possibility is that what we measure in both data sets is correct, but our model is incorrect. The photometry and astrometry data arise from the same physical process, meaning the brightness and position must be consistent. So, something is missing there.”

Both teams also estimated the velocity of the super-compact lensing object. The Lu/Lam team found a relatively sedate speed, less than 30 kilometers per second. The STScI team found an unusually large velocity, 45 km/s, which is the result of an extra kick that the purported black hole got from the supernova that generated it.

Journal Reference:

  1. Casey Y. Lam, Jessica R. Lu et al. An isolated mass gap black hole or neutron star detected with astrometric microlensing. DOI: 10.48550/arXiv.2202.01903
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